The war began on 25 June 1950 when North Korea invaded South Korea following clashes along the border and insurrections in the south.

After the surrender of Japan at the end of World War II, Korea was divided into two zones of occupation. The Soviets administered the northern zone and the Americans administered the southern zone. In 1948, as U.S. and Soviet forces began to withdraw, the occupation zones became two sovereign states. But the governments of the two new Korean states each claimed to be the sole legitimate government of all of Korea, and neither accepted the border as permanent.

On 10 August 1945, U.S. Colonels Dean Rusk and Charles H. Bonesteel III were assigned the task of dividing Korea into Soviet and U.S. occupation zones. They proposed the 38th Parallel as the dividing line. This was incorporated into the US General Order No. 1 which responded to the Japanese surrender on 15 August. Explaining the choice of the 38th Parallel, Rusk observed, "even though it was further north than could be realistically reached by U.S. forces, in the event of Soviet disagreement ... we felt it important to include the capital of Korea in the area of responsibility of American troops".

The Jeju uprising was notable for its extreme violence. Between 14,000 and 30,000 people (10% of Jeju's population) were killed, and 40,000 fled to Japan. In 2006, almost 60 years after the Jeju uprising, the South Korean government apologized for its role in the killings and promised reparations.

In 1949, North Korean leader Kim Il-sung began seeking the Soviet Union's support for an invasion, hoping to unite Korea after its division by foreign powers. In April 1950, Joseph Stalin gave Kim permission to attack the government in the South under the condition that China would agree to send reinforcements if needed. Kim met with Mao Zedong in May 1950, and although Mao was concerned the U.S. would intervene, he agreed to support the North Korean invasion because China desperately needed the economic and military aid promised by the Soviets.

The Truman Doctrine was an American foreign policy with the primary goal of containing Soviet geopolitical expansion during the Cold War. Truman told Congress that "it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures." He contended that because totalitarian regimes coerced free peoples, they automatically represented a threat to international peace and the national security of the United States.

Twenty-one countries of the United Nations eventually contributed to the UN force, with the United States providing around 90% of the military personnel.

In the U.S., the war was initially described by President Harry S. Truman as a "police action" as the United States never formally declared war on its opponents and the operation was conducted under the auspices of the United Nations.

Both Buzz Aldrin and Neil Armstrong piloted planes in Korea, with Aldrin completing 66 missions and Armstrong completing 78.

An astonishing 38 percent of U.S. prisoners died in captivity. Life as a POW meant many forced marches in subfreezing weather, solitary confinement, brutal punishments and attempts at political "re-education." Torture was common and the Geneva Convention was not followed. More than 7,100 Americans were captured and imprisoned and just over 2,700 are known to have died while imprisoned.